1. Field of the Invention
The present invention relates to vapor compression systems and, more particularly, to a transcritical multi-stage vapor compression system in which the efficiency and capacity can be adjusted.
2. Description of the Related Art
Vapor compression systems are used in a variety of applications including heat pump, air conditioning, and refrigeration systems. Such systems typically employ working fluids, or refrigerants, that remain below their critical pressure throughout the entire vapor compression cycle. Some vapor compression systems, however, such as those employing carbon dioxide as the refrigerant, typically operate as transcritical systems wherein the refrigerant is compressed to a pressure exceeding its critical pressure and wherein the suction pressure of the refrigerant is less than the critical pressure of the refrigerant. The basic structure of such a system includes a compressor for compressing the refrigerant to a pressure that exceeds its critical pressure. Heat is then removed from the refrigerant in a first heat exchanger, e.g., a gas cooler. The pressure of the refrigerant discharged from the gas cooler is reduced in an expansion device and then converted to a vapor in a second heat exchanger, e.g., an evaporator, before being returned to the compressor.
FIG. 1 illustrates a typical transcritical vapor compression system 10. A compression mechanism 12 compresses the refrigerant from a suction pressure to discharge pressure that exceeds the critical pressure of the refrigerant. The refrigerant is then cooled in a gas cooler 14. In the illustrated example, a suction line heat exchanger 16 further cools the high pressure refrigerant before the pressure of the refrigerant is reduced by expansion device 18. The refrigerant then enters evaporator 20 where it is boiled and cools a secondary medium, such as air, that may be used, for example, to cool a refrigerated cabinet. The refrigerant discharged from the evaporator 20 passes through the suction line heat exchanger 16 where it absorbs thermal energy from the high pressure refrigerant before entering the compression mechanism to repeat the cycle.
In practice, the cooling load requirement varies during the operation of the refrigeration system. If the refrigeration system operates with excess cooling capacity, then its efficiency is less than optimal. The cooling capacity, and thus the efficiency, of a carbon dioxide refrigeration system can be modulated by varying the gas cooler pressure. The pressure of the carbon dioxide refrigerant in the gas cooler is a parameter that is very important to the efficiency and cooling capacity of the refrigeration system. The amount of carbon dioxide refrigerant in components operating at the supercritical pressure, e.g., gas cooler 14 and suction line heat exchanger 16, varies with the gas cooler pressure. However, the amount of refrigerant existing at any instant in time in the evaporator and other components of the refrigeration system varies little with changes in gas cooler pressure. The difference between carbon dioxide and other refrigerants is that the density of the supercritical carbon dioxide varies much more than the density of commonly used subcritical refrigerants that are in liquid phase in the condenser and the suction line heat exchanger.
It is desirable to ensure constant thermodynamic parameters, such as temperature and pressure, at the suction of the first stage compressor. Those parameters are used in designing the evaporator, suction line heat exchanger, thermostatic expansion valves, compressor internal heat transfer, oil separator, accumulator, etc. A constant temperature and pressure for the vapors entering the compressor results in the vapors entering the compressor having a constant density. The constant density of the vapors, in turn, results in the compressor having a fixed displacement volume and pumping with a constant mass flow rate. The design of the various components of the refrigeration system is simplified by these parameters being constant.
What is needed in the art is an apparatus for adjusting the efficiency and capacity of a transcritical multi-stage vapor compression system, while keeping the pressure and temperature at the input of the compressor constant.